Dependence on the interlayer spacing of nanoscale electronic states in layered nitride chloride superconductors α- Mx TiNCl investigated by scanning tunneling microscopy/spectroscopy

We studied the influence of interlayer spacing on the nanoscale electronic states of layered nitride chloride superconductors α-MxTiNCl (M = pyridine (Py), Na, K) using scanning tunneling microscopy and spectroscopy. It was shown that these materials exhibit a strong decrease in the critical temperature (Tc) with increasing interlayer spacing d. Our measurements revealed gap-like features in differential conductance spectra that remain nearly independent of Tc and d. The extracted characteristic gap energy Eg shows spatial inhomogeneity on nanometer scales and does not follow the conventional Bardeen–Cooper–Schrieffer scaling. These observations suggest that the detected gap-like feature is more consistent with pseudogap-like behavior, most probably related to other electronic properties, than with true superconducting pairing. It is similar to the situation in cuprate high-temperature superconductors, where pseudogap is a prominent feature. The results suggest the importance of the local electronic structure and doping effects in determining superconducting properties of layered systems.